Apple: Fix mapping CMAKE_APPLE_ARCH_SYSROOTS to custom OSX_ARCHITECTURES

The `CMAKE_OSX_ARCHITECTURES` value is not used directly by generators.
It is used to initialize a per-target `OSX_ARCHITECTURES` property, but
that property can also be set explicitly by project code to a subset of
the full list of architectures.  In order to handle this case, construct
a mapping from each `CMAKE_OSX_ARCHITECTURES` entry to the corresponding
`CMAKE_APPLE_ARCH_SYSROOTS` entry by name.  Use the mapping to find the
sysroot for each entry in `OSX_ARCHITECTURES` for a given target.

If `CMAKE_APPLE_ARCH_SYSROOTS` does not have the same length as
`CMAKE_OSX_ARCHITECTURES`, error out early rather than risking a crash
or assertion failure.

Fixes: #20534

Source/cmLocalGenerator.h

@@ -513,6 +513,7 @@ protected:
   std::map<std::string, std::string> VariableMappings;
   std::string CompilerSysroot;
   std::string LinkerSysroot;
+  std::unordered_map<std::string, std::string> AppleArchSysroots;
 
   bool EmitUniversalBinaryFlags;